Upon binding a progestational hormone, the progesterone hormone undergoes an activation process which transforms it from a cytoplasmic to a nuclear-binding form. In the chick oviduct, dissociation of the receptor into subunits (seen as a shift in sedimentation coefficient from 8S to 4S) appears to be concurrent with activation. The removal of a small inhibitor of activation has been suggested. It will be determined whether the shift in sedimentation coefficient is simply coincidental with activation or activation indeed consists of dissociation of the native receptor into its component subunits. The inhibitor will be characterized once the activated receptor has been translocated to the nucleus, there is a gradual replenishment of the non-activated form in the cytoplasm. The course of replenishment and molecular characteristics of the regenerated cytoplasmic receptor will be studied, using any antibody to the native progesterone receptor.